1. Field of the Invention
The present invention relates to a method for treatment of tissue, for example, collagenous tissue, where a deleterious mechanical loading environment contributes to the degradation of the tissue. In one embodiment, the present invention relates to a method for treatment of degenerated intervertebral discs to improve fatigue resistance, and to non-toxic crosslinking reagents that are effective fatigue inhibitors.
In a second embodiment, the present invention relates to methods and devices for the treatment of intervertebral disc and surrounding tissues to resist the ongoing deforming forces and curve progression in scoliosis.
In a third embodiment, the present invention relates to methods and devices for improving the environment for biological activity in the central region of the disc by increasing the permeability or more specifically, the diffusivity of the outer region of the disc.
2. Description of the Related Art
Deleterious mechanical loading environments contribute to the degradation of collagenous tissue in a variety of manners. For instance, fatigue is a weakening of a material due to repetitive applied stress. Fatigue failure is simply a failure where repetitive stresses have weakened a material such that it fails below the original ultimate stress level. In bone, two processes—biological repair and fatigue—are in opposition, and repair generally dominates. In the intervertebral disc, the prevalence of mechanical degradation of the posterior annulus (Osti 1992) suggests that fatigue is the dominant process. Active tissue response (adaptation, repair) does not play a strong role in the case of mature intervertebral disc annular material. As a principally avascular structure, the disc relies on diffusion for nutrition of its limited number of viable cells. Age related changes interfere with diffusion presumably contributing to declining cell viability and biosynthetic function (Buckwalter et al. 1993, Buckwalter 1995). Age related decline in numbers of cells and cell functionality compromises the ability of the cells to repair mechanical damage to the matrix. While regeneration of the matrix in the nucleus following enzymatic degradation has been accomplished, albeit inconsistently (Deutman 1992), regeneration of functional annular material has not yet been realized.
Combined with this limited potential for repair or regeneration, studies have shown that posterior intervertebral disc tissue is vulnerable to degradation and fatigue failure when subjected to non-traumatic, physiologic cyclic loads. Prior work has shown deterioration in elastic-plastic (Hedman 99) and viscoelastic (Hedman 00) material properties in posterior intervertebral disc tissue subjected to moderate physiological cyclic loading. Cyclic load magnitudes of 30% of ultimate tensile strength produced significant deterioration of material properties with as little as 2000 cycles. Green (1993) investigated the ultimate tensile strength and fatigue life of matched pairs of outer annulus specimens. They found that fatigue failure could occur in less than 10,000 cycles when the vertical tensile cyclic peak exceeded 45% of the ultimate tensile stress of the matched pair control. In addition, Panjabi et al (1996) found that single cycle sub-failure strains to anterior cruciate ligaments of the knee alter the elastic characteristics (load-deformation) of the ligament. Osti (1992) found that annular tears and fissures were predominantly found in the posterolateral regions of the discs. Adams (1982) demonstrated the propensity of slightly degenerated discs to prolapse posteriorly when hyperflexed and showed that fatigue failure might occur in lumbar discs as the outer posterior annulus is overstretched in the vertical direction while severely loaded in flexion. In an analytical study, interlaminar shear stresses, which can produce delaminations, have been found to be highest in the posterolateral regions of the disc (Goel 1995). These prior data indicate: 1) the posterior disc and posterior longitudinal ligament are at risk of degenerative changes, and that 2) the mechanism of degeneration can involve flexion fatigue.
A different type of mechanical degradation of collagenous tissue occurs in scoliosis. Scoliosis refers to an abnormal lateral, primarily, or other curvature or deformity of the spine. Severe curvature and ongoing curve progression can lead to many other health disorders including but not limited to compromised respiratory function. In addition, one's lifestyle can be adversely affected and a loss of cosmesis can result. A large segment of the population is affected by scoliosis, approximately 2% of women and 0.5% of men. Over 80% of scoliosis is of no known origin (i.e., idiopathic). Approximately 80% of idiopathic scoliosis develops in young pubescent adults. Existing conservative approaches to limit curve progression can be awkward or restricting, and are of disputed value. Surgical correction of severe curves can be intensive with a long recovery period, require the need for post-operative bracing, and be fraught with many other post-operative problems.
Current treatments for scoliosis consist of bracing and surgery. The purpose of orthopaedic braces is to prevent increasing spinal deformity, not to correct existing deformity. Braces are generally used in children with an expected amount of skeletal growth remaining, who have curve magnitudes in the range of 25 to 40 degrees. External braces are routinely used as a standard of care. Yet there is controversy regarding the effectiveness of external bracing for scoliosis. The magnitude of forces delivered to the spine corresponding to brace loads applied to the torso cannot be quantified directly. Larger forces applied to the torso may also result in brace induced pathologies to the tissues in contact with the brace. Some studies suggest that braces are effective in halting curve progression in about 80 percent of afflicted children. But because the option to do nothing but observe curve progression is inappropriate, there is no generally accepted percentage of these curves that would stop progressing on their own or due to other factors.
Naturally occurring collagen crosslinks play an important role in stabilizing collagenous tissues and, in particular, the intervertebral disc. Significantly higher quantities of reducible (newly formed) crosslinks have been found on the convex sides than on the concave sides of scoliotic discs (Duance, et al. 1998). Similarly, Greve, et al. (1988) found a statistically increased amount of reducible crosslinks in scoliotic chicken discs at the same time that curvatures were increasing. This suggests that there is some form of natural, cell-mediated crosslink augmentation that occurs in response to the elevated tensile environment on the convex side of scoliotic discs. Greve also found that there were fewer reducible crosslinks at the very early stages of development in the cartilage of scoliotic chickens. They concluded that differences in collagen crosslinking did not appear to be causative because there was not a smaller number of crosslinks at later stages of development. In fact, later on, when the scoliotic curve was progressing, there were statistically significant greater numbers of collagen crosslinks, perhaps in response to the curvature. Although not the conclusion of Greve, this can be interpreted as being a sufficient depletion of crosslinks in the developmental process with long enough duration to trigger the progression of scoliotic curvature that was later mended by a cellular response that produced higher than normal levels of crosslinks. These studies suggest that the presence of naturally occurring collagen crosslinks may be critical to prevent ongoing degradation and for mechanical stability of intervertebral disc tissue in scoliotic spines.
It is well documented that endogenous (naturally occurring—enzymatically derived and age increasing non-enzymatic) and exogenous collagen crosslinks increase the strength and stiffness of collagenous, load-supporting tissues (Wang 1994, Chachra 1996, Sung 1999a, Zeeman 1999, Chen 2001). Sung (1999b) found that a naturally occurring crosslinking agent, genipin, provided greater ultimate tensile strength and toughness when compared with other crosslinking reagents. Genipin also demonstrated significantly less cytotoxicity compared to other more commonly used crosslinking agents. With regard to viscoelastic properties, Lee (1989) found that aldehyde fixation reduced stress-relaxation and creep in bovine pericardium. Recently, naturally occurring collagen crosslinks were described as providing ‘sacrificial bonds’ that both protect tissue and dissipate energy (Thompson, et al. 2001). To date, there is no known reference in the literature as to the ability of exogenous crosslinks to decrease the viscoelastic characteristic of hysteresis or to increase the ability of the collagenous tissue to store energy. A need therefore exists to find biochemical methods that enhance the body's own efforts to stabilize discs in scoliotic spines by increasing collagen crosslinks.
Mechanical degradation of collagenous tissue can also occur if the environment for biological activity in the central region of the disc is poor. Tissue engineering is a burgeoning field which aims to utilize cells, special proteins called cytokines and synthetic and native matrices or scaffolds in the repair and regeneration of degraded, injured or otherwise failed tissues. With regard to the intervertebral disc, biological solutions like tissue engineering are hindered by the harsh, avascular (very little if any direct blood supply) environment of moderately degenerated intervertebral discs. The disc is known to receive nutrients and discard cell waste products primarily by diffusion through the annulus fibrosus and through the cartilaginous endplates that connect the disc to the bony, well vascularized, spinal vertebrae. The disc cartilaginous endplates loose permeability by calcification while the disc itself becomes clogged up with an accumulation of degraded matrix molecules and cell waste products. This loss of disc permeability effectively reduces the flow of nutrients to the cells in the interior central region of the disc, the nucleus pulposus. This loss of flow of nutrition to the disc causes a loss of cell functionality, cell senescence, and causes a fall in pH levels that further compromises cell function and may cause cell death (Buckwalter 1995, Horner and Urban 2001). Horner and Urban showed that density of viable cells was regulated by nutritional constraints such that a decline in glucose supply led to a decrease in viable cells. Boyd-White and Williams (1996) showed that crosslinking of basement membranes increased permeability of the membranes to macromolecules such as serum albumin, crosslinked albumin, and a series of fluorescein isothiocyanate dextrans of four different molecular sizes. It is reasonable to assume, then, that increased crosslinking of the annulus fibrosus of intervertebral discs would provide for increased flow of glucose to cells in the interior region of the disc, thus improving their viability.
To date, however, no treatments capable of reducing mechanical degradation to collagenous tissues currently exist. In fact, no other collagenous tissue fatigue inhibitors have been proposed. A need therefore exists for a method for improving the resistance of collagenous tissues in the human body to fatigue and for otherwise reducing the mechanical degradation of human collagenous tissues, in particular, the posterior annulus region of the intervertebral disc. In addition, a need exists to increase resistance to scoliotic curve progression by treatment of appropriate regions on the tensile side (convex) of scoliotic discs and to improve permeability throughout the whole disc annulus and the flow of nutrition to cells in the central portion of the disc.
Additional advantages and novel features of this invention shall be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following specification or may be learned by the practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentalities, combinations, compositions, methods, devices, and application trays particularly pointed out in the appended claims.